When viewing a scene using a traditional digital imaging sensor or by eye, the intensity of light from each point or pixel of the imaged scene can be determined for each of three wavelength bands (centred around red, green and blue for a digital camera, and yellowish-green, green and bluish-violet for the human eye). Information about the full spectral emissions (i.e. a continuous graph of intensity over wavelength) of the scene can, at best, be represented in only a three-dimension colour space, necessitating a loss of information.
Multispectral sensors have been used in research into aquatic (freshwater, brackish water and salt water) environments for about 30 years. Multispectral sensors are divided into more than three discrete colour bands and so give more detailed spectral information. They typically have a minimum wavelength resolution of 10 nm. They have typically been carried in satellites, aeroplanes, buoys and boats to analyse upwelling radiance remotely, and in underwater vehicles to measure both upwelling and downwelling radiance in situ. In both cases the light measured by the sensor comes from natural illumination that is incident on the water.
Hyperspectral sensors are also known. These have a much better wavelength resolution than multispectral sensors—at least 10 nm or better and can operate over a broad range of wavelengths including visible light and typically also into ultraviolet and infrared frequencies. It is also known to use hyperspectral sensors for imaging purposes in passive remote sensing. A hyperspectral imager (also known as an imaging spectrometer, imaging spectroscope, imaging spectroradiometer, superspectral or ultraspectral imager), is capable of determining the light intensity from each point or pixel of a scene for each of a large number (typically hundreds) of wavelength bands, each no more than 10 nm wide. This results in far more spectral information about the scene being preserved than is the case when just three bands are available, as for conventional imaging.
Because hyperspectral imagers give such detailed spectral information for each pixel in the image, independently of each other, it is possible to identify regions containing particular types of matter, such as chemical substances and organisms, by using their known unique spectra.
Applications for hyperspectral imagers include mineral exploration, agriculture, astronomy and environmental monitoring. They are typically used in aeroplanes (so-called “remote” viewing).
An overview of the use of hyperspectral sensors in oceanography is given is “The New Age of Hyperspectral Oceanography” by Chang et al. in Oceanography, June 2004, pp. 23-29. WO 2005/054799 discloses the use of a hyperspectral imager from airborne platforms to observe coastal marine environments remotely. The use of an airborne hyperspectral imager for mapping kelp forest distribution close to the shore is described in “Kelp forest mapping by use of airborne hyperspectral imager” by Volent et al. in Journal of Applied Remote Sensing, Vol. 1, 011503 (2007).
However, the applicant has realised that taking hyperspectral images remotely from the air or from space has several limitations. For example, even for very optically clear water, such as can be found in the Arctic, it is not possible to distinguish features of the sea bottom or of suspended matter beyond a depth of a few meters. In more typical marine waters, even this limited visibility is drastically reduced and is normally less than a meter or so—in murkier waters maybe only a few centimeters might be penetrable by light. This limits the usefulness of this technique. Additional problems occur due to Interference from the air between the water surface and the remote imager; for example, due to clouds and to Rayleigh scattering. It is also necessary to take into account the angle of the sun in the sky. Furthermore, the spatial resolution of conventional remote sensing systems, such as a hyperspectral imager mounted in an aeroplane, is typically relatively low.